It is somewhat ironic that population pressure, a key force in the origins of agriculture, has also occasionally led to the collapse of agricultural economies and cultures. Some would even say that this is happening today, although agricultural production seems to have kept pace with human population growth so far. That was not always the case, as archaeologists have seen from a number of places around the globe.
Today the Fertile Crescent is not particularly fertile. Part of the reason for the decline in productivity of the region was human intervention, through destructive agricultural practices that denuded the land: overgrazing, cutting wood for fuel, erosion, and so on.23 But there is also a close correlation between climatic downturns around 4,200 years ago and the abandonment of the rain-fed lands of northern Mesopotamia.24 Many northern people moved to southern Mesopotamia, where irrigation fed the fields. But that put greater stress on the land — irrigation leads to salinization of the soil. Year after year of pouring water over the land adds salts, and erodes nutrients.
Apparently, people also became stressed. The inhabitants of southern
Mesopotamia built a wall to exclude the hungry refugees of the north from entering their lands. Ultimately the society collapsed.
Peter deMenocal of Columbia University chronicled the changes many agricultural communities of the past endured in the face of climatic downturns.25 Even the collapse of the great Mayan civilization appears to have been related to a combination of climate change and population pressure.26 In each case, large areas became depopulated; only in some cases did nature eventually restore the land.
In some places the ravages of agriculture made it difficult for nature to restore the previous ecosystem. Salinization of the soil from irrigation is just one example. Even weeds have problems reestablishing nature's services to such lands. We can also change local climates: overgrazing of lands in the Sahel of Africa led to reduced rainfall and expansion of the desert.
Ohio was a region that saw a full recovery after an agricultural collapse. Atop a bluff downstream from my "office," the rock in the Olentangy River where I sit and think, are the remains of a fortress established about a thousand years ago. It attests to the large, successful populations of Native Americans that once dominated the region, growing corn, beans, and squash. But when Europeans first entered Ohio in the seventeenth century, they encountered a heavily wooded area with few inhabitants.27 What had happened?
Part of the problem was the nasty side of humanity. As populations grew and productive lands became scarce, the protohistoric peoples of Ohio and the surrounding area turned their bows and arrows toward each other. Hence the palisades around many of the ancient villages. Indeed, the bluff-top fortress site 110 feet above the Olentangy River presents more than just an attractive vista: it also provides a commanding view for the defense of people's territory. In the protohistoric period, it appears that expansion of the Iroquois nation from the northeast led to raids on the Shawnee populations of Ohio. But that alone cannot explain the abandonment of this rich region.
Population pressure also brings disease. Just as tightly packed monocultures of crops are subject to evolving pathogens, so are people. Thus another part of the protohistoric Ohio population problem, once again evidenced by skeletal remains, was overreliance on corn. The resulting malnutrition not only had its own direct effects, but reduced human resistance to bacteria and viruses. Moreover, Europeans brought diseases to which native Americans had no immunity.28 Millions died from tuberculosis and other diseases — that is truly how the West was "won." But the Ohio region had been largely depopulated before those devastating epidemics spread across the continent.
The final assault on early Ohio populations was famine. A drought one year would leave too little food to store. A flood in another year could wipe out the crops planted along fertile river banks. Although the native Ohioans were learning to deal with the unpredictability of nature, the ecological transition was not far enough along for them to have adequate control.
The archaeological record in Ohio and around the world reveals just what Thomas Malthus envisioned as the consequences of population growth: war, famine, and disease. The agricultural revolution was just one phase along the way. As much as we like to romanticize Native Americans living in perfect harmony with their environments, the archaeological record shows otherwise. From the overkill of megafauna in the early years to the subtle impacts of agriculture on wild species of plants and animals, as long as there were enough people, earth's biodiversity has had to give way to growing human populations. In some cases the expansion led to continuing extinctions of other species; in other cases there were setbacks, and humans had to give way to nature.
The perceived balance that Native Americans had with nature was due in part to lack of numbers: their populations had not yet grown to the extent of those in other parts of the world. Whereas native Americans grew to sufficient numbers to bring down a number of large animal species, the rest of the world had a head start at population growth and began spilling over into the Americas. What the Europeans found was a land that had not been transformed to the same degree as lands of the Old World.
Brown University anthropologist Shepard Krech put it this way: "The native people who molded North America were fully capable of transformative action in ecosystems they knew intimately, but in almost all instances their populations were too small to have made much of a difference. And when people few in number quickly became fewer from disease, the lands they had burned, cleared, and planted — lands transformed and exploited for purposes relating to agriculture, fuel, hunting, gathering, construction, and other ends—rested and recovered from whatever human pressures they had been under."29 Given a few more years of population growth in the Americas, rather than decline from European diseases, the land may have looked very different upon the arrival of people from the Old World.
One irony of population pressure is that it led to the preservation of a large mammal species in at least one case. The bison that were once bountiful across the American plains were a favored target for Native American hunters. As the human population grew, the number of bison dwindled. But because population pressure can also lead to human aggression and war, there were often buffer zones on territorial boundaries between warring tribes. Thus, when Lewis and Clark took their famous expedition across the American West, they found that bison were plentiful only in the intertribal buffer zones, where the beasts were free to roam without human predators.30 This of course changed when the expansion of European populations across the country led to wasteful slaughters of bison herds, pushing them to the brink of extinction.
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